Machine tools of various types, controlled by automatic machining control circuit means such as computers or microprocessors, are now widely utilized. Such machine tools offer considerable advantages with respect to enhancing automation of manufacturing processes with reduced manpower being required. However with prior art types of such machine tool, the automatic machining control circuit means are not capable of detecting when the cutting tool fitted to the machine tool has been damaged, and so will not perform proper machining and may in fact damage the workpiece. There is therefore a requirement for means to sense such tool damage in an automatic manner, and to use the results of such sensing to notify the operating staff by an audible and/or visible warning signal, and moreover to act on the automatic machining control circuit means such as to cause correct countermeasures (e.g. immediate halt of machining operations) to be automatically initiated.
Such tool damage can be sensed in a variety of ways, for example by detecting changes in vibrational acceleration, increased feed drag, etc. However the level of acoustic emission (sometimes abbreviated to AE) generated when a damaged cutting tool is first brought into contact with the workpiece (i.e. when machining feed begins) has been found to provide a highly accurate indication of tool damage.
If only a single type of machining condition is to be anticipated, i.e. only a single type of cutting tool will be used, with a single type of material to be machined, at a fixed feed rate, then it is sufficient simply to convert the energy of the acoustic emission into an electrical signal, to compare this signal level with a predetermined fixed value corresponding to the maximum amount of acoustic emission to be expected so long as there is not cutting tool damage, and generating an alarm signal if this predetermined value is exceeded. However, the level of acoustic emission which is generated during machining operations using an undamaged tool, although substantially constant for any particular combination of machining parameters, can vary substantially between different sets of parameters. That is to say, the "normal" level of acoustic emission for machining using one type of cutting tool may be much higher than the level produced when machining using a different type of cutting tool, and may in fact exceed the acoustic emission level produced for the condition of tool damage when using the latter tool. Thus, if only a single fixed value is used as a detection level for sensing a state of tool damage, it will not be possible to perform damage detection for a variety of different machining conditions.
There is therefore a requirement for an apparatus for sensing damage to the cutting tool of an automatically controlled machine tool, which is applicable to a variety of operating conditions of the machine tool, and it is one object of the present invention to provide such an apparatus.
In addition, various types of electrically or mechanically generated noise may be produced at times other than when machining is in progress, and such noise may be sensed as a high level of acoustic emission, and therefore result in generation of spurious tool damage warning signals. The elimination of such erroneous warning signals is another objective of the present invention.